A periphery circumstance of a vehicle changes from time to time during the vehicle's travelling, and such periphery circumstance puts a variety of psychological and physical burden (so-called a driving burden) on a driver. An increase of the driving burden may possibly have a negative effect on a driving operation. Accordingly, attempts to eliminate such a negative effect are being made by estimating a driving burden and reflecting the estimated driving burden on various types of process executed by in-vehicle systems.
A technique to use an inter-vehicle time has been proposed recently as a technique of estimating a driving burden (Patent Literature 1). The inter-vehicle time is a time taken until a subject vehicle reaches a present location of a preceding vehicle (inter-vehicle distance/traveling speed of the subject vehicle). By estimating a driving burden using the inter-vehicle time, a driving burden from the preceding vehicle can be estimated.